The invention relates to carrier subscriber telephone systems in which the subscriber terminal includes a local battery that is charged with line current from the central office battery, and more particularly to a control circuit for automatically disabling the local battery charging circuit for a prescribed time interval in response to high voltage and low voltage conditions on the line.
The U.S. Pat. No. 4,145,572, issued Mar. 20, 1979, entitled "Power Supply Control Circuit for Subscriber Carrier Telephone System" by James A. Stewart discloses a control circuit that shuts off a charging circuit associated with a local battery in a carrier subscriber terminal on a cable pair when an associated physical handset is off hook and when central office routiner equipment is connected to the cable pair to check for extraneous voltages and leakages on the subscriber loop. The routiner equipment makes a dc leakage or line insulation test on the cable pair, and thus the control circuit, by charging a capacitor to a prescribed voltage, connecting the capacitor across the cable pair, and measuring the capacitor voltage after a prescribed time interval. This is also a measure of the input impedance of the control circuit and the current drawn by this circuit. The input impedance of and control current drawn by a control circuit according to the patent are 240K ohms and 200 microamperes when 48 volts dc is connected across the line. System specifications have recently been established which enable ALIT (Automatic Line Insulation Test) equipment to indicate changes in line leakage sooner (i.e., the trouble threshold was raised). By way of example, if the threshold is set at 100K ohms and the control circuit represents only 240K ohms, then trouble tickets are printed for lines (carrying subscriber carrier channels) with less than 171K ohms leakage, whereas trouble tickets are printed for physical subscriber circuit lines (without carrier on it) with less than 100K ohms leakage. If the leakage represented by the carrier channel is increased to 800K ohms, however, then trouble tickets for lines with subscriber carrier are printed when the actual combined line leakage is greater than 114K ohms, which is close to that for a line without subscriber carrier. It is desirable, therefore, that the input impedance of the control circuit be as high as possible.
An object of this invention is the provision of improved circuitry for selectively disabling the local battery charging circuit of a carrier subscriber station.